The present invention is directed to a method of catalyzing the process for de-blocking blocked isocyanates to form crosslinked coatings. More particularly, the present invention relates to the use of certain bismuth carboxylates that are effective in catalyzing both a solvent borne and a waterborne process to form such crosslinked coatings.
The bismuth carboxylates of the present invention are useful at a very low concentration of 0.05-0.5 wt % of metal based on the total resin weight and are highly effective. This is very desirable in that the emission into the environment of bismuth, a low toxicity metal, is reduced to very low levels.
Blocked isocyanates have been used in many coating applications, such as powder coatings, electrocoatings, coil coatings, wire coatings, automotive clear topcoatings, stone chip resistant primers, and textile finishes. Traditionally, these coating processes employ organic solvents, which may be toxic and/or obnoxious and cause air pollution. In recent years, the legal requirements for low or no pollution of the environment have led to an increase in the interest in waterborne and high solids coatings.
In processes wherein blocked isocyanates are used, heating to an elevated temperature is necessary to remove the blocking group from the blocked isocyanate to form free isocyanates. The free isocyanates then react with polyols (polymers containing hydroxy functional groups) to form a crosslinked network as a thin film coating. An obstacle to the use of this process is the high temperature required to remove the blocking group. The process is extremely slow without a catalyst. It is known that metal compounds such dialkyltin and certain bismuth and zinc salts are excellent catalysts in these solvent borne coating processes. xe2x80x9cCrosslinking with Polyurethanes.xe2x80x9d W. J. Blank, ACS Proceedings of Polymeric Materials Science and Engineering(1990) 63:931-935.
Bismuth organo-compounds have been used in a variety of processes wherein polyisocyanates or blocked isocyanates is an ingredient. For example, EP 95-109602 describes an epoxide amine adduct with a bismuth compound as being useful in a conventional cationic coating process. U.S. Pat. No. 5,702,581 describes the use of organic bismuth complexes in phosphate dip coating compositions to provide corrosion resistance. The bismuth organic complexes include bismuth carboxylates, such as bismuth lactate. WO 95/29007 disclosed the use of bismuth compounds/mercapto complexes for curing polyisocyanate organic solvent compositions. The bismuth compounds disclosed include bismuth carboxylates, nitrates and halides. WO 96/20967 also described bismuth/zinc mixture with a mercapto complex as a catalyst for producing polyurethane. See also Frisch et al., xe2x80x9cNovel Delayed-Action Catalyst/Co-catalyst system for C.A.S.E. Applicationsxe2x80x9d, 60 Years Polyurethanes, Kresta et al. ed., Technomic: Lancaster, Pa. 1998, pp. 287-303. Further, WO 95/08579 described bismuth/mercapto complexes as latent catalysts in a polyol-polyisocyanate adhesive system. The catalyst is described as useful in promoting the rapid cure of the system. The bismuth carboxylates described in these references are those wherein the carboxylate has ten carbons or less in the hydrocarbon structure. These conventional bismuth carboxylates do not provide improved resin performance nor are they effective in water-borne formulations.
WO 95/07377 described the use of bismuth lactate in cationic lacquer compositions, which employ urethane reactions. A mixture of bismuth and an amino acid or amino acid precursor was disclosed for catalyzing a cationic electrodeposition of a resin film on a metal substrate. The bismuth may be present in the form of nitrates, oxides, trioxides, or hydroxide. DE 19,532,294A1 also disclosed bismuth carboxylates as catalysts for single component polyurethane lacquer coatings in a solvent borne formulation.
Unfortunately, when the known bismuth catalysts are employed in waterborne coatings formulations, it was found that they were not effective. It is suspected that the loss of activity is related to the hydrolysis of the bismuth salt in water. Moreover, even if these compounds function as catalysts in waterborne processes, it has been our experience that a very high level is necessary, usually 10 to 100 times higher than in solvent borne processes. This is undesirable because bismuth has a low degree of toxicity and would cause environmental pollution if a large amount is released into the environment.
Bismuth carboxylates have been used as catalysts in processes that do not involve de-blocking of blocked isocyanates. Bismuth dimethylol propionate has been disclosed in DE 93-43,300,002 as being useful in an electrocoating process for coating phosphate dipped metals to provide anti-corrosion and weather resistance. Bismuth carboxylates are also described in DE 96-19,618,825 for use in an adhesive gel formulation that is safe for contact with human skin. The formulation contains polyether polyols with hydroxy groups, antioxidants, Bismuth(III) C2-C18 carboxylates soluble in the polyether polyols and OCN(CH2)6NCO. JP 95-351,412 describes the use of bismuth neodecanoate as a catalyst for two part adhesive formulations containing polyisocyanates, polyols with an ethylenediamine. These formulations do not involve the de-blocking of blocked isocyanates.
For waterborne processes, the catalysts known to be useful are organo-tin and lead compounds. See WO 95/04093, which describes the use of organo-tin alone or in a mixture with other compounds including bismuth oxide in a low temperature curing process employing blocked isocyanates. There is no disclosure of bismuth carboxylates alone as a catalyst for de-blocking isocyanates. Organo-tin compounds have also been used in coatings, e.g. in paints for anti-fouling applications. Organo-tin compounds in mixtures with bismuth hydroxy carboxylic acid salt was described in DE19,613,685. The use of bismuth lower carboxylates was described as being useful in a phosphate dip process to provide corrosion resistance to lacquer coatings. The bismuth carboxylates described therein as being useful are lower carboxylate of bismuth wherein the carboxylic acid has up to ten carbons. The substrate is then coated with an epoxy resin in the presence of a blocked isocyanate as the crosslinking agent using a zinc organo compound and/or lead compound as the catalyst. EP0,509,437 disclosed a mixture of a dibutyltin aromatic carboxylate with a bismuth and a zirconium compound as the dissociation catalyst for electrocoating wherein a blocked isocyanate is used. Polystannoxane catalysts are also described in EPO,810,245 A1 as an low temperature catalyst for curing compositions comprising a blocked isocyanate. Bismuth compounds, including carboxylates were described as being useful as a co-catalyst. However, the process is one in which the reaction temperature was in the range of 100xc2x0 C., quite a bit below the normal temperature of 120xc2x0 C. to 150xc2x0 C. for de-blocking blocked polyisocyanates. JP 94-194950 described a formulation for coating materials which are rapidly curable in contact with an amine catalyst vapor or mist. The coating formulation included polyols, polyisocyanates, antimony or bismuth catalysts with mercaptans in an organic solvent. The toxicity of both lead and tin compounds present serious environmental hazards The use of solvents in solvent borne processes further result in the undesirable release of toxic and obnoxious chemicals into the environment. For these reasons, the use of organo tin and lead compounds and solvents has been banned in many applications and is highly restricted in electrocoating.
It is, therefore, important to develop other catalysts or catalysts systems for waterborne processes.
Certain bismuth carboxylates have now been developed as effective catalysts for coatings processes wherein a temperature of at least 130xc2x0 C. is used to de-block blocked isocyanate employed as the crosslinking agent. The bismuth carboxylates of the present invention are compounds wherein the carboxylate is derived from a carboxylic acid with more than ten carbon atoms. The bismuth carboxylates of the present invention have been found to be highly effective and efficient catalysts and are useful at very low concentrations both in solvent and water-borne processes. Thus, even though bismuth has a low toxicity, the low concentrations required reduces the emission of bismuth into the environment and thus reduces environmental contamination.
An objective of the present invention is to develop catalysts, which are effective, and efficient for de-blocking blocked isocyanate to form crosslinked coatings with reduced environmental contamination.
It is a second objective of the present invention to develop catalysts, which may be used at very low concentration levels to reduce the emission of toxic substances into the environment.
It is a further objective to develop a catalyst, which is highly effective not only in solvent borne processes, but retains its high effectiveness in waterborne coating compositions.
It is another objective to reduce the emission of metals into the environment.
In accordance with the present invention a process has been developed for forming blocked isocyanate crosslinked coatings formulations comprising as a catalyst, a bismuth carboxylate, wherein the carboxylate is derived from a carboxylic acid with a linear or branched hydrocarbon chain of 11-36 carbons. The hydrocarbon chain may contain heteroatoms such as nitrogen, oxygen or sulfur. The carboxylic acid may be aromatic or aliphatic, such as: undecanoic, dodecanenoic, palmitic, stearic, oleic, isostearic, abitiec acids. The carboxylic acid may be synthesized or derived from natural fats. The catalyst may also comprise a mixture of the carboxylic acids useful in the present invention.
In accordance with the present invention a catalyst for crosslinked blocked isocyanate coatings has been developed. The catalyst comprise a bismuth salt of a hydrophobic carboxylic acid having 11-36 carbon atoms in the hydrocarbon chain and a molecular weight in the range of 165-465. Preferably the carboxylic acid suitable for the present invention is water-insoluble.
The carboxylic acid salts of bismuth of the present invention is prepared by heating 1M of bismuth trioxide, Bi2O3, with at least 3M of a carboxylic acid or an anhydride and removing the water from the reaction mixture. The reaction is carried out at a temperature of 120-150xc2x0 C. for several hours or until a clear solution is obtained. The bismuth carboxylates of the present invention are known to be unstable in the presence of water. They are hydrolyzed and break down into their constituent parts: bismuth oxide and the carboxylic acid from which they are formed. However, surprisingly, on heating to 130xc2x0 C., the bismuth carboxylate reforms and is highly effective as a catalyst.
This catalyst is also effective in solvent borne coatings, which contain low levels of water, such as in pigmented coatings. Furthermore, the bismuth carboxylates of the present invention are stable and have a longer shelf life.
The catalytic activity of the bismuth catalysts useful in the present invention can be maintained if the salt is prepared from a hydrophobic, carboxylic acid having 11-36 carbons in the hydrocarbon chain. The hydrocarbon chain may comprise non-carbon atoms, such as nitrogen, oxygen or sulfur. The carboxylic acid may be aromatic or aliphatic with a molecular weight in the range of 165-465. The carboxylic acids useful in the present invention are water-insoluble. In a water-borne electrocoating process, the cationic resin codeposit with the hydrolyzed bismuth carboxylate of the present invention as a mixture of bismuth oxide and carboxylic acid. When heated to a curing temperature of 130xc2x0 C. or above, the bismuth carboxylate forms in situ to catalyze the process.
The carboxylic acid salts of bismuth may be prepared by heating one mole of bismuth trioxide (Bi2O3) with at least 3 moles of a carboxylic acid or an anhydride and removing the water formed. The reaction is carried out at a temperature of 120-150xc2x0 C. for 1 to 5 hours or until a clear solution is obtained. The bismuth carboxylate catalysts of the present invention are effective in both solvent borne and water borne coatings. It is particularly useful in pigmented coatings.
The bismuth carboxylates of the invention may be used singly or in combination as mixtures. To form a mixture of the carboxylates, a mixture of carboxylic acids or anhydrides may be used in the process. The carboxylates may also be prepared in situ, i.e. a mixture of a bismuth salt such as bismuth oxide or chloride with carboxylic acid, singly or in mixtures may also be incorporated into the coating formulation.
Cationic water-borne resins or cationic electrocoating resins useful in this invention can be typically prepared by reacting a bisphenol A type epoxy resin with an epoxy equivalent weight of between 200 to 2000, preferably between 400 to 1000 with an amine. The amine can be ammonia, a secondary, primary or a tertiary amine. If ammonia is used in the preparation of the cationic resin, the reaction of the epoxy resin with ammonia has to be conducted in the presence of large excess of free ammonia to suppress gelation of the resin. In this reaction a combination of primary, secondary and tertiary amine functional resin is formed. With primary amines, depending on the ratio of amine to epoxy secondary, and tertiary amine functional resins are formed. With secondary amines tertiary amine functional resins are produced. If an excess of epoxy is used and if the reaction is conducted in the presence of some water and neutralizing acid, there is also the potential for the formation of quaternary ammonium group containing resins.
Another way to prepare cationic resins is by co-polymerization of cationic monomers such as dimethyl-amino-propyl-methacrylate, dimethyl-amino-ethyl-methacrylate, dimethyl-amino-propyl-acrylamide or t-butyl-amino-ethyl-acrylate with an acrylic or methacrylic ester monomer or optionally with styrene or acrylonitrile. Other methods are the reaction of anhydride functional polymers with amines with primary or secondary and t-amine groups and a mono epoxide compound as shown in U.S. Pat. No. 3,984,382.
If a waterborne formulation is desired, an alcohol or a polyol can be solubilized or dispersed in water in the presence of nonionic groups or a nonionic surfactant. The alcohol or polyol may be incorporated in the bisphenol epoxy resin itself. For example, a bisphenol epoxy resin can be reacted with a methoxy-polyethylene glycol or a methoxy-polyethylene-ether-amine with a MW of between 500 to 2000.
Waterborne resin formulations suitable for this invention may also include resins dispersed in water in the presence of a nonionic surfactant. An epoxy or an acrylic or polyester resin may be dispersed in water. The nonionic groups can be a part of the resin structure or a part of an external surfactant. Commercial products, which are suitable, include a dispersion in water of solid bisphenol A glycidyl resins with a molecular weight of between 900 to 4000.
The blocked isocyanate crosslinker useful in this invention are aromatic or aliphatic isocyanates with a blocking group, which can be removed. Often the de-blocking to the isocyanate is a displacement reaction, wherein the blocking group is displaced with another group. Typical blocking groups for the isocyanate are selected from the group consisting of malonates, triazoles, xcex5-caprolactam, phenols, ketoxime, pyrazoles, alcohols, glycols, glycol ethers and uretdiones.
Some typical di or polyisocyanates suitable for the invention are: hexamethylene diisocyanate, isocyanurate trimer, biuret, isophorone diisocyanate, tetramethylxylidine diisocyanate and methylene bis(phenyl isocyanate). Typical examples of blocking groups are methyl ethyl ketoxime, xcex5-caprolactam, 1,2,4-triazole, 3,5-dimethylpyrazole, phenol, 1,2-ethylene glycol, 1,2-propylene glycol, 2-ethylhexanol, 2-butoxyethanol, 2-methoxy (2-ethoxy ethanol).
The cationic resins suitable for the invention may also be typically dispersed in water in the presence of a suitable water soluble organic acid such as formic, acetic, glycolic or lactic acid or an inorganic acid such as sulfamic acid.
A coating formulation is normally prepared by blending and dispersing the blocked isocyanate crosslinker, the cationic resin and the catalyst of this invention in water. If pigments are added they can be dispersed separately in the resin. If neutralization of the cationic resin with an organic acid is required, the acid can be added to the resin or to the water phase. Usually high shear dispersers are used to emulsify or disperse the resin.
The catalyst of this invention is also advantageous for use in solvent borne coating formulations. Most pigmented formulation have shown a decrease of catalytic activity on aging. This reduction in catalyst activity is attributable to the presence of water on the surface of the pigment. Based on experience, it is known that catalyst deactivation takes place if the coating formulations are cured at high humidity. Although the catalyst of the present invention is predominately suitable for waterborne or water-dispersed coatings. It would be advantageous to use a catalyst with a long shelf life in solvent borne coatings, such as a catalyst of the present invention.
The present invention further includes a coating formulation comprising of polyol, a blocked isocyanate crosslinker and a bismuth carboxylate catalyst. The coating formulation can contain varying amounts of water, depending on the desired application. The bismuth carboxylates of the present invention is a salt of bismuth and a carboxylic acid with a total of between 11 to 36 carbons or a mixture of bismuth oxide or chloride with a carboxylic acid with a total of between 11 to 36 carbons. The carboxylates of the present invention are soluble in a water-immiscible solvent.
The present invention is further directed to a cationic electrocoating formulation comprising a water-dispersible cationic polyol, a blocked isocyanate and a bismuth catalyst of the present invention.
The water-dispersible cationic polyol is at least di-functional, preferably tri functional or higher. The blocked isocyanate is present at a molar ratio sufficient to facilitate crosslinking. The bismuth catalyst is used at a concentration of between 0.01 to 0.5 weight percent (wt %), preferably between 0.1 to 1.0 wt %, of metal based on the total resin solids in the formulation.
The catalyst may also comprise a bismuth salt, such as an oxide or chloride and not a carboxylate, in combination with a free carboxylic acid to form the bismuth carboxylate of the present invention in situ. The bismuth carboxylate may be made from a single or a mixture of carboxylic acids.
Typical carboxylic acids suitable for the present invention are selected from the group consisting of linear and branched, saturated and unsaturated, aromatic and cycloaliphatic C11 to C36 mono and di carboxylic acids. Examples of suitable carboxylic acids include undecanoic, dodecanenoic, palmitic, stearic, oleic, isostearic, abetiec acid, etc. These acids can be derived from natural fats or produced synthetically.
The present invention further include coating compositions comprising a polyol, a blocked isocyanate and a bismuth salt of a water insoluble aliphatic, cycloaliphatic or aromatic, carboxylic acid with a total number of carbons in the hydrocarbon chain of between 11 to 36. Optionally, the chain may contain heteroatoms such as oxygen, nitrogen or sulfur. The catalyst of the present invention also includes blends of bismuth salts, such as bismuth oxides and chlorides with the above acids, or a mixture of the bismuth carboxylates as defined above.
The bismuth carboxylate of the present invention is added to the resin blend at a concentration of between 0.01 to 1.0 wt %, preferably between 0.05 to 0.5 wt %, of bismuth metal based on the total resin weight in the formulation. Preferably the resin is in a waterborne cationic electrocoating formulation system.